www.biogeosciences.net/13/4899/2016/ doi:10.5194/bg-13-4899-2016
© Author(s) 2016. CC Attribution 3.0 License.
Source, transport and fate of soil organic matter inferred from
microbial biomarker lipids on the East Siberian Arctic Shelf
Juliane Bischoff1,a, Robert B. Sparkes2,b, Ayça Do˘grul Selver2,3, Robert G. M. Spencer4, Örjan Gustafsson5, Igor P. Semiletov6,7,8, Oleg V. Dudarev7,8, Dirk Wagner9, Elizaveta Rivkina10, Bart E. van Dongen2, and Helen M. Talbot11School of Civil Engineering and Geosciences, Newcastle University, Newcastle upon Tyne, UK
2School of Earth and Environmental Sciences and Williamson Research Centre for Molecular Environmental Science, University of Manchester, Manchester, UK
3Balıkesir University, Geological Engineering Department, Balıkesir, Turkey
4Earth, Ocean and Atmospheric Science, Florida State University, Tallahassee, FL, USA
5Department of Environmental Science and Analytical Chemistry (ACES) and the Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden
6Pacific Oceanological Institute, Far Eastern Branch of the Russian Academy of Sciences, Vladivostok, Russia 7International Arctic Research Center, University of Alaska, Fairbanks, USA
8National Tomsk Research Polytechnic University, Tomsk, Russia
9GFZ German Research Centre for Geosciences, Helmholtz Centre Potsdam, Section 5.3 Geomicrobiology, Telegrafenberg, Potsdam, Germany
10Institute of Physicochemical and Biological Problems in Soil Science, Russian Academy of Sciences, Pushchino, Russia anow at: The Lyell Centre, Heriot-Watt University, Edinburgh, UK
bnow at: School of Science and the Environment, Manchester Metropolitan University, Manchester, UK
Correspondence to:J. Bischoff (j.bischoff@hw.ac.uk) and B. E. van Dongen (bart.vandongen@manchester.ac.uk) Received: 6 April 2016 – Published in Biogeosciences Discuss.: 15 April 2016
Revised: 8 August 2016 – Accepted: 10 August 2016 – Published: 6 September 2016
Abstract. The Siberian Arctic contains a globally significant pool of organic carbon (OC) vulnerable to enhanced warm-ing and subsequent release by both fluvial and coastal ero-sion processes. However, the rate of release, its behaviour in the Arctic Ocean and vulnerability to remineralisation is poorly understood. Here we combine new measurements of microbial biohopanoids including adenosylhopane, a lipid associated with soil microbial communities, with published glycerol dialkyl glycerol tetraethers (GDGTs) and bulk δ13C measurements to improve knowledge of the fate of OC trans-ported to the East Siberian Arctic Shelf (ESAS). The micro-bial hopanoid-based soil OC proxy R0soil ranges from 0.0 to 0.8 across the ESAS, with highest values nearshore and decreases offshore. Across the shelf R0soil displays a nega-tive linear correlation with bulk δ13C measurements (r2= −0.73, p =< 0.001). When compared to the GDGT-based OC proxy, the branched and isoprenoid tetraether (BIT)
in-dex, a decoupled (non-linear) behaviour on the shelf was ob-served, particularly in the Buor-Khaya Bay, where the R0
soil shows limited variation, whereas the BIT index shows a rapid decline moving away from the Lena River outflow channels. This reflects a balance between delivery and removal of OC from different sources. The good correlation between the hopanoid and bulk terrestrial signal suggests a broad range of hopanoid sources, both fluvial and via coastal erosion, whilst GDGTs appear to be primarily sourced via fluvial transport. Analysis of ice complex deposits (ICDs) revealed an aver-age R0soil of 0.5 for the Lena Delta, equivalent to that of the Buor-Khaya Bay sediments, whilst ICDs from further east showed higher values (0.6–0.85). Although R0soil correlates more closely with bulk OC than the BIT, our understanding of the endmembers of this system is clearly still incomplete, with variations between the different East Siberian Arctic re-gions potentially reflecting differences in environmental
ditions (e.g. temperature, pH), but other physiological con-trols on microbial bacteriohopanepolyol (BHP) production under psychrophilic conditions are as yet unknown.
1 Introduction
The Arctic permafrost region is a globally significant or-ganic carbon (OC) pool containing approximately 1300 Pg (uncertainty range ∼ 1100 to 1500 Pg) of carbon. Approxi-mately 800 Pg (60 %) is stored below the ground in frozen permafrost, with the remainder (∼ 500 Pg) occurring in non-permafrost soils, seasonally thawed in the active layer or in deeper taliks (Hugelius et al., 2014). Permafrost is defined as ground (soil or rock and includes ice and organic mate-rial) that remains below 0◦C for at least two consecutive years (van Everdingen, 2005) and is naturally particularly sensitive to an increase in global temperatures. It is there-fore a focal point of ongoing climate change research on the observed (e.g. Romanovsky et al., 2010) and predicted rise in atmospheric and soil temperature (IPCC, 2013). Rising temperatures in the Arctic are causing, amongst other severe consequences for society and infrastructure, shifts in hydro-logical processes and progressive deepening and duration of permafrost thawing during the Arctic summer (Vonk et al., 2015). This destabilisation of permafrost deposits will in-crease the re-distribution of terrestrial organic matter (OM) to the Arctic Shelf and ultimately the Arctic Ocean by (1) transportation via the major Arctic rivers and (2) erosion of coastal areas.
The Arctic Ocean receives around 10 % of the global river discharge, while representing only 1 % of the global ocean water body (Opsahl et al., 1999). Climate change has already increased the water discharge to the Arctic Ocean (Peterson et al., 2002; Rawlins et al., 2010). Arc-tic rivers are distinct in their hydrologic regime with pro-nounced seasonality (Holmes et al., 2012, 2013). They dis-charge the majority of their annual load of water, sedi-ment and total OC from May to July (Dittmar and Kat-tner, 2003; Holmes et al., 2012, 2013). The drainage basins of these rivers include areas of continuous and discontinu-ous permafrost (Feng et al., 2015; Gustafsson et al., 2011; Kotlyakov and Khromova, 2002; and references therein). Thawing of permafrost deposits is linked to a destabilisa-tion of stored carbon by top-down thawing at the active-layer–permafrost interface leading to collapse of ice-rich per-mafrost, also known as thermokarst, resulting in hydrologi-cal changes (Vonk et al., 2015). Thermokarst processes in-cluding massive erosional events can lead to increased mo-bility of old carbon (both dissolved and particulate) from the lower layers and strongly affects the balance of carbon dioxide (CO2) and methane (CH4) emissions from these en-vironments (e.g. Gustafsson et al., 2011; Schuur et al., 2009; Vonk et al., 2015). For example, between 1985 and 2004 an
increase in the proportion of mobilised terrestrial OC ac-counted for by ancient carbon of 3–6 % has been estimated (Feng et al., 2013). Not only will this increased transport of older material increase the release of CO2to the atmosphere (Drake et al., 2015; Mann et al., 2015; Spencer et al., 2015) but already observed increases in river discharge will also lead to increased terrestrial OC input into the Arctic Ocean (Savelieva et al., 2000; Semiletov et al., 2000, 2013). How-ever, the fate of this terrestrial OC in the Arctic Ocean system is not well understood.
Additionally, OC is stored, frozen, within coastal ice com-plex deposits (ICDs) – this can also be a major source of terrestrial OC to the Arctic Ocean (Lantuit et al., 2013). These deposits erode at a rate greater than that of temperate coasts with an average rate for the Arctic coast of 0.5 m yr−1, albeit with high local variability, up to 10 m yr−1 (Lantuit et al., 2013). The highest rates are found in the Laptev, East Siberian and Beaufort seas, where the majority of the coast comprises frozen unlithified material highly suscepti-ble to erosion (Fig. 1; Lantuit et al., 2012, 2013). Although at present much of the unlithified coast is located in areas still largely protected by sea ice, continuing decline in sea ice extent will expose this material to erosion and increase sediment flux to the ocean (Lantuit et al., 2013). The rela-tive contribution of permafrost ICD erosion to sedimentary carbon in the East Siberian Arctic Sea (ESAS) is estimated to be 57 ± 1.6 % (Vonk et al., 2012). Other recent estimates vary widely but suggest that between 15 and 66 % of this carbon is remineralised to CO2 acting as a positive feed-back for climate warming, whilst the remainder is buried in shelf sediments (Knoblauch et al., 2013; Vonk et al., 2012). Furthermore, Tesi et al. (2016) showed that the potential for burial and degradation of terrestrial OC on the ESAS, based on the different chemical reactivity of different components, is dependent on the source material (topsoil vs. ICD) and the transportation pathways (river run-off vs. coastal erosion), highlighting the need to better understand these sources.
A number of methods are used to trace different primary sources of sedimentary OM on the Arctic shelves includ-ing bulk δ13C, δ15N and C / N, as well as molecular ratios (Cooke et al., 2009; Drenzek et al., 2007; Feng et al., 2013, 2015; Goñi et al., 2005; Guo et al., 2004; Gustafsson et al., 2011; Semiletov, 1999a, b; Tesi et al., 2014; van Dongen et al., 2008; Vonk et al., 2012). The molecular ratios include the branched and isoprenoid tetraether (BIT) index (Hop-mans et al., 2004), based on comparison of branched glycerol dialkyl glycerol tetraethers (brGDGTs) from terrestrial soil environments and the marine isoprenoid GDGT crenarchaeol (e.g. De Jonge et al., 2014, 2015; Do˘grul Selver et al., 2015; Sparkes et al., 2015), and the bacteriohopanepolyol (BHP)-based R0soilindex (De Jonge et al., 2016; Do˘grul Selver et al., 2012, 2015).
BHPs are microbial membrane lipids comprising penta-cyclic triterpenoids with an extended polyfunctionalised side chain (Rohmer, 1993; see Table S1 for structures). They
oc-! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !!! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! # * # * # * # * # * # * CH OR CR KY KI CB 160° E 140° E 75 ° N 70 ° N #
*Ice complex s amples
!ISSS-08 Coastal erosion >1 m y-1 Arctic Ocean Laptev Sea East Siberian Sea Buor-Khaya Bay Dmitry Laptev Strait Ko lym a ou tflow Lena River
Yana River Indigirka River
Kolyma River L e n a o ffs h o re Ind igirk a ou tflow ESAS ffshore
ESAS offshore
ESAS earshore
ESAS nearshore
Figure 1. Map of the East Siberian Arctic Shelf (ESAS) showing sampling stations of the International Siberian Shelf Studies 2008 (ISSS-08) expedition and location of Ice complex deposit (ICD) samples investigated in this study. Key regions discussed in the text are highlighted. Lower courses and outflows of four great Russian Arctic rivers are labelled. Section of coastline indicated in red are areas of moderate to high
rates of coastal erosion (> 1 m yr−1) as defined by Lantuit et al. (2011). Key: KI, Kurungnakh Island; CB, Cape Bykovsky; KY, Indigirka
(Tesi et al., 2014); CR, Chukochya River; OR, Omolon River; CH, Cherskii (Tesi et al., 2014).
cur in varying concentrations and compositions in a range of environmental settings such as Arctic permafrost soils (Höfle et al., 2015; Rethemeyer et al., 2010), lakes (Coolen et al., 2008; Talbot and Farrimond, 2007; Talbot et al., 2003c), and marine sediments (e.g. Blumenberg et al., 2009, 2010, 2013; Cooke et al., 2009; Do˘grul Selver et al., 2012; Talbot et al., 2014; Wagner et al., 2014; Zhu et al., 2011). Recent studies have indicated the potential of a specific group of BHPs with a cyclised side chain to be used as a tracer for soil organic matter (SOM) input in aquatic settings. Adenosylhopane (1a; see Table S1 for structures; Bradley et al., 2010, and refer-ences therein), two related structures with yet undetermined terminal groups termed “adenosylhopane type 2” (1b; Cooke et al., 2008a) and “adenosylhopane type 3” (1b’; Rethemeyer et al., 2010) together with their C-2 methylated homologues (2a, 2b and 2b’, respectively) are common compounds in soils (Cooke et al., 2008a; Kim et al., 2011; Rethemeyer et al., 2010; Spencer-Jones et al., 2015; Xu et al., 2009; Zhu et al., 2011). However, these compounds are rarely found in marine settings with the exception of deep-sea fan systems, which comprise a significant proportion of terrestrial OM in-cluding BHPs (Cooke et al., 2008b; Handley et al., 2010; Wagner et al., 2014). A BHP-based SOM proxy, the Rsoil index, was proposed in which the relative proportion of soil marker BHPs (adenosylhopane and related compounds) to the combined total of soil markers plus the commonly oc-curring BHP bacteriohopane-32,33,34,35-tetrol (BHT, 1f) is calculated (Zhu et al., 2011). The use of BHT in this context is complicated as it is also found in varying proportions in
soils but is frequently the most significant and, in some cases, only BHP in marine sediments, hence its proposal as the only possible representative compound for marine OM-dominated sediments (De Jonge et al., 2016; Zhu et al., 2011).
The Rsoil index has been investigated in various settings, including the Yangtze River–East China Sea surface sedi-ment transect (Zhu et al., 2011) and several (sub-)Arctic land to ocean transects (De Jonge et al., 2016; Do˘grul Selver et al., 2012, 2015). These studies showed that it can be used to trace SOM exported from land to ocean. However, due to the lim-ited and intermittent occurrence of methylated compounds in the sub-Arctic setting, Rsoil(Eq. 1) was modified to R0soil for application in Arctic settings where the C-2 methylated soil markers are scarce and therefore were excluded (Eq. 2; Do˘grul Selver et al., 2012, 2015).
Rsoil= soil BHPs(1a + 1b + 1b’ + 2a + 2b + 2b’) soil BHPs(1a + 1b + 1b’ + 2a + 2b + 2b’) + BHT(1f) (1) R0soil= soil BHPs(1a + 1b + 1b’) soil BHPs(1a + 1b + 1b’) + BHT(1f) (2)
This study focuses on the ESAS, a region dominated by fluvial input from three major rivers, namely the Lena, Indi-girka and Kolyma (Gordeev, 2006), as well as a site of signif-icant erosion of coastal ICD (Fig. 1; Lantuit et al., 2012). Re-cently, Sparkes et al. (2015) used the GDGT-based BIT proxy to trace terrestrial OM on the ESAS shelf using the same sediment samples and found a decoupling between BIT and bulk δ13C, suggesting that GDGTs were (primarily) sourced
via riverine transport and not from erosion of coastal ICD. In addition, a strong linear correlation between bulk δ13C and R0
soil and a strong but non-linear relationship between the BIT index and R0soil was observed by Do˘grul Selver et al. (2015) in surface sediments along the offshore transect off the Kolyma River. This suggests a decoupling between these microbial-based biomarker proxies and different and/or additional sources of BHPs to the ESAS compared to the GDGTs. Erosion of ICD was proposed as a likely source for the BHPs, suggesting that these biomarker proxies probably reflect different OC sources (Do˘grul Selver et al., 2015). It has, however, been suggested that coastal cliffs could also be a source of branched GDGTs, at least in sites without major river inputs (De Jonge et al., 2015). At this time, it remains unclear (i) whether the decoupling between these bacterial biomarker-based proxies is unique to the Kolyma region or more widely applicable to the whole ESAS and (ii) whether the soil marker BHP pool has a mixed input from ICD and river-transported OC or can be used as a proxy for ICD.
Therefore, this study investigates the abundance and com-position of terrestrial microbial (soil marker) BHPs across the land–ocean transect of the ESAS in conjunction with the recently published BIT data obtained at the same sites (Sparkes et al., 2015). This study includes new data on BHPs in ICD from the Lena Delta and Indigirka and Kolyma river-banks in order to further constrain the source of OC trans-ported to and deposited into the Arctic Ocean.
2 Materials and methods
2.1 Study area and sample collection
This study focuses on the ESAS with sediment samples from the Laptev Sea, Buor-Khaya Bay, Dmitry Laptev Strait and East Siberian Sea (Fig. 1). Comprehensive fieldwork was conducted in August–September 2008 as part of the Inter-national Siberian Shelf Studies 2008 expedition (ISSS 08; Semiletov and Gustafsson, 2009). Surface sediments were recovered using a dual gravity corer or a van Veen grab sampler from H/V Yakob Smirnitskyi (ESAS) and TB-0012 (Buor-Khaya Bay). The sediment samples were transferred with stainless steel spatulas to polyethylene containers and frozen at −18◦C for transport and storage (Karlsson et al., 2011). Subsamples were taken and freeze-dried for subse-quent total lipid extraction (Sparkes et al., 2015).
The sediments investigated in this study were grouped based on their location on the ESAS (Fig. 1; Table S2; Sparkes et al., 2015). Samples were grouped longitudinally, into the Buor-Khaya Bay and associated region offshore of the Lena River delta (the Laptev Sea), the Dmitry Laptev Strait (the narrow channel between the coastline at ∼ 140◦E and the New Siberian Islands, splitting the ESAS up into two distinct areas – the Laptev Sea and the East Siberian Sea), the region offshore of the Indigirka River mouth and the
re-gion offshore of the Kolyma River mouth. The Indigirka and Kolyma River mouth offshore regions are generally equiv-alent to the western and eastern East Siberian Sea regions, respectively, as identified by Semiletov et al. (2005). The ESAS samples have also been classified latitudinally, into the nearshore ESAS (< 150 km from river outflows) and off-shore ESAS (> 150 km from river outflows).
In addition to surface sediment samples throughout the ESAS, this study also includes ICD samples from loca-tions on the Siberian mainland, including the central Lena Delta, Cape Bykovsky, and the Kolyma and Indigirka river banks. The site on Kurungnakh Island (central Lena Delta; 72◦200N, 126◦170E) was drilled during the Russian–German LENA 2002 expedition in July 2002 (Bischoff et al., 2013) and a 24 m long permafrost core from a low-centred ice-wedge polygon was recovered (Grigoriev, 2003). In total 23 samples from depths 0.34 to 24.55 m (Table S3) were cho-sen for BHP analysis. An additional ice complex sample (CB IC 1.9; Table S3) was obtained from Cape Bykovsky. The Bykovsky Peninsula is located in the vicinity of the Lena Delta in an area of significant coastal erosion (Lantuit et al., 2011). Ice complex samples from the Kolyma region were obtained from the Chukochya River (CR; Fig. 1), which out-flows in the Kolyma Gulf; the Omolon River (OR; Fig. 1), a tributary of the Kolyma River; and from Cherskii (CH, Fig. 1; Table S3; Tesi et al., 2014). An additional profile from the Indigirka watershed is also included for comparison (KY, Fig. 1; Tesi et al., 2014).
2.2 Bulk analysis
Data for the ISSS-08 sediments are taken from Karlsson et al. (2011, see Table S2). Total organic carbon (TOC) data for the Indigirka and Cherskii profiles are taken from Tesi et al. (2014) (see Table S3). The Cape Bykovsky, Chukochya River and Omolon River permafrost ICD samples were prepared for carbon isotope analysis according to Harris et al. (2001) and analysed at the University of California, Davis, Stable Isotope Facility.
2.3 Extraction of ESAS sediment samples
Sediment samples were extracted using a modified Bligh– Dyer method as described in more detail in Do˘grul Selver et al. (2015) and Sparkes et al. (2015). Briefly, sediment (5 g) was ultrasonically extracted using a monophasic mix-ture of methanol/dichloromethane/phosphate buffer (0.05 M, pH 7.4; 2 : 1 : 0.8 v : v : v). The supernatant was separated by centrifugation and the remaining sediments re-extracted twice. The combined organic phases were evaporated to dry-ness. After extraction, the total lipid extracts (TLEs) were re-dissolved in dichloromethane/methanol (2 : 1) and sepa-rated into fractions with one-sixth of the TLE used for BHP analysis.
2.4 Extraction of ice complex samples
Freeze-dried and ground ICD samples were extracted us-ing a modified Bligh–Dyer method (1959) that was adapted from the method described in Cooke et al. (2008a). Briefly, samples (∼ 3 g, dry weight) were ultrasonically extracted with a monophasic mixture of methanol/chloroform/water (10 : 5 : 4 v : v : v). Deviating from the method described in Cooke et al. (2008a), the sonication steps were reduced to 30 min at 40◦C without the subsequent overnight shaking. The supernatant was removed after centrifugation (16 000 g, 10 min) and the remaining sediment was re-extracted twice. After phase separation via addition of water (5 mL) and chlo-roform (5 mL), the organic phases were combined, evapo-rated to dryness, and blown to dryness under N2. Extracts were then re-dissolved in 2 : 1 chloroform/methanol and one-third of the TLE was used for BHP analyses.
2.5 Solid phase extraction and derivatisation of BHPs Aliquots (one-sixth for sediments and one-third for terrestrial materials) of the TLE were loaded onto NH2solid phase ex-traction (SPE) cartridges pre-conditioned with 6 mL of hex-ane (1 g / 6 mL; Isolute, Biotage, Sweden), in 200 µL chlo-roform and separated into two fractions (Fr): Fr. 1 (non-polar + acidic, 6 mL of diethylether/acetic acid (98 : 2, v : v)) and Fr. 2 (polar, 12 mL of methanol), which contained all BHPs except 32,35-anhydroBHT (e.g. Bednarczyk et al., 2005). After separation, the internal standard (5α-pregnane-3β,20β-diol) was added to Fr. 2 and dried under N2. This SPE method was adapted from a method commonly used in other studies of complex polar lipids from environmen-tal samples (e.g. Lupascu et al., 2014). Fr. 2 was acety-lated with pyridine/acetic anhydride (1 : 1, v : v; 500 µL) for 1 h at 50◦C and left at room temperature overnight. The samples were evaporated to dryness, re-dissolved in methanol/propan-2-ol (60 : 40, v : v) and filtered through a 0.2 µm PTFE syringe filter. For BHP analysis, the sam-ples were dissolved in methanol/propan-2-ol (60 : 40, v : v; 500 µL). Sample injection volume was 10 µL.
2.6 Analytical HPLC-APCI-MS
BHPs were identified and measured using reverse-phase HPLC-APCI-MS as previously described in Cooke et al. (2008a). Chromatographic separation was achieved under the conditions described in van Winden et al. (2012b). BHP structures were identified based on previously published spectra (Cooke et al., 2008a; Rethemeyer et al., 2010; Tal-bot and Farrimond, 2007; TalTal-bot et al., 2003a, b). Semi-quantitative estimation of BHP concentrations was achieved by employing the characteristic base peak ion areas of indi-vidual BHPs in mass chromatograms (from SCAN 1) relative to the m/z 345 chromatogram base peak area of the acety-lated 5α-pregnane-3β,20β-diol internal standard. Averaged
relative response factors relative to the internal standard, de-termined from a suite of acetylated BHP standards, were used to adjust the BHP peak areas where N-containing compounds give an average response 12 times that of the standard and compounds without N 8 times that of the standard (for fur-ther details see van Winden et al., 2012b). The reproducibil-ity of triplicate injections was 3–6 % RSD (standard error: ±1–4 µg g−1OC) for BHT and 5–8 % RSD (standard error: ±1– 2 µg g−1OC) for adenosylhopane in the environmental samples, resulting in an absolute standard error of on average ±0.01 for R0soil(see Sect. 1, Eq. 2).
3 Results and discussion
OC concentrations and bulk carbon isotopes for sediments recovered throughout the ISSS-08 expedition have been re-ported previously (Vonk et al., 2012; Karlsson et al., 2011). OC concentrations ranged from 0.68 to 2.25 wt. % C and were highest in Buor-Khaya Bay, with values for the differ-ent regions shown in Table S2 (see also Sparkes et al., 2015). Bulk δ13C values ranged from −21.2 to −27.4 ‰ with most depleted values reported in sediments in Dmitry Laptev Strait and ESAS nearshore sediments.
3.1 BHP concentrations and distributions in ESAS surface sediments
In total, 92 surface sediment samples throughout the ESAS in Buor-Khaya Bay, Dmitry Laptev Strait, and Kolyma River and Indigirka River mouth transects were analysed for their BHP composition (Table S2). Up to 16 individual BHPs were identified in the ESAS sediments, with the total concentra-tion of BHPs ranging from 12 to 824 µg g−1OC (Table S2). However, their concentrations and distributions differed with distance to the mainland throughout the shelf. BHT (1f) was the most abundant single BHP, ranging from 9 to 313 µg g−1OC (Table S1, Figs. 2a and 3a). The relative proportion of BHT was lowest in Buor-Khaya Bay sediments close to the main-land (mean = 37 % of all detected BHPs) rising to 80 % (mean = 65 %) of all detected BHPs in the ESAS offshore sediments furthest away from the mainland. Highest BHT concentrations were measured closest to the mainland ex-cept in the region of the Indigirka outflow (samples YS-26 to 30; Table S2), where values were lower than in all other nearshore settings (Fig. 2a). However, mean concentra-tions were stable with increasing distance from the mainland (Fig. 3a) and considerable amounts of BHT (up to 77 µg g−1OC) are still detectable at 293 km offshore.
In addition to the ubiquitous and abundant BHT (1f), a suite of other polyhydroxylated BHPs related to BHT were detected, including the BHT isomer (1f’), which has been linked to production by pelagic anaerobic or-ganisms performing anaerobic ammonium oxidation (an-namox; Rush et al., 2014). The C-2 methylated homologue
2-Figure 2. Maps of (a) BHT and (b) summed non-methylated soil
marker concentrations (µg g−1OC) and (c) the resulting R0soilin ISSS
sediments from the ESAS. Maps were interpolated using a kriging algorithm (ArcGIS v.10; ESRI Ltd) and the locations of the ISSS-08 stations are shown as black dots.
MeBHT (2f), unsaturated BHT (16-1f) and bacteriohopane-30,31,32,33,34,35-hexol (1g) were also common, especially in Buor-Khaya Bay, although at much lower concentration than BHT (Table S2). Soil marker BHPs identified included high proportions of adenosylhopane (1a), followed by adeno-sylhopane type 2 (1b) and adenoadeno-sylhopane type 3 (1b’). The soil marker type 2 and 3 compounds are related to adenosyl-hopane but have different and as yet uncharacterised termi-nal groups compared to adenosylhopane as identified by LC-MSn (Table S1; Rethemeyer et al., 2010). The C-2 methy-lated soil markers (2a, 2b, 2b’; Table S1) were present
inter-Figure 3. Box plots summarising the concentrations of (a) BHT, (b)
summed non-methylated soil markers and (c) the resulting R0soilon
the ESAS, grouped by distance from river mouths. Concentrations in ice complex samples are also shown (see Fig. 1 for locations). Thick lines show the median values, boxes the 25th and 75th per-centiles, whiskers the maximum and minimum values within 1.5 times the interquartile range, and circular symbols outliers beyond this threshold.
mittently and always at lower concentration than the corre-sponding non-methylated structures. Generally, the concen-trations of all non-methylated soil markers are highest in samples closer to the coast (0–100 km) and decrease with distance from the river outflows (Figs. 2b and 3b; Table S2) showing similar trends.
Sediments of Buor-Khaya Bay and Dmitry Laptev Strait were characterised by high amounts of adenosylhopane, with a mean average of 64 µg g−1OC(range 7–137 µg g−1OC; Table S2) and total non-methylated soil markers accounted for up to 66 % of total BHPs although the mean proportion was lower at 36 %. Sediments collected from the ESAS offshore region contain noticeably lower soil marker BHPs both in absolute and relative concentrations with non-methylated soil mark-ers ranging from 0 to 62 µg g−1OC(mean 11 % of all detected BHPs; Table S2; Figs. 2b and 3b). Methylated compounds were frequently below the detection limit (Table S2) as previ-ously reported in Arctic and sub-Arctic River mouth surface sediment transects (Do˘grul Selver et al., 2012, 2015).
Total concentrations of non-methylated soil markers in ESAS sediments with a range of 0–218 µg g−1OC are similar to sediments from the Yenisei River system, including the Yenisei River mouth, gulf, outflow and nearby Kara Sea (9– 508 µg g−1OC; De Jonge et al., 2016). The highest values oc-curred in the Yenisei River mouth sediments and also in Khalmyer Bay (329–435 µg g−1OC), a nearby area of coastal erosion, although not directly drained by the Yenisei River itself. However, in the Yenisei mouth, Khalmyer Bay, Buor-Khaya Bay and Dmitry Laptev Strait, non-methylated soil markers had very similar mean relative abundance as a pro-portion of total BHPs of 30–40 %. This reflects a significant contribution of these terrestrial compounds to the total BHP assemblage in Arctic settings (see also Cooke et al., 2009; Taylor and Harvey, 2011) and indicates a strong terrestrial signal in the Arctic Shelf sediments. The proportion of to-tal soil markers in sediments from the outflow of other non-Arctic rivers has been shown to be somewhat lower (e.g. <20 % from the estuary and seaward of the Yangtze River, Zhu et al., 2011; < 8 % Congo River estuary, Spencer-Jones et al., 2015), thus emphasising the importance of obtaining values for local endmembers. The higher abundance of these highly functionalised compounds in Arctic sediments may be a result of better preservation under the cold-temperature conditions of this region. Additionally, temperature may also influence the microbiological community, leading to deliber-ate accumulation of adenosylhopane and/or limited biosyn-thetic transformation of the BHP precursor in this extreme environment (see Rethemeyer et al., 2010).
Other BHPs identified include three structures with amine functional groups at the C-35 position. The concentration of aminotriol (1e; Table S1) varied from 0 to 53 µg g−1OC (mean 6.4 % of all analysed BHPs) throughout the ESAS (Table S2). Aminotetrol (1d) was generally less abundant across the ESAS (0–13 µg g−1OC) and aminopentol (1c) was identified in 37 of the 47 Buor-Khaya Bay sediments and only occasion-ally in other areas (Table S2). Aminotetrol and aminopen-tol in particular have been linked to aerobic methane oxi-dising bacterial sources and have been proposed as mark-ers for terrestrial methane oxidation in continental wetlands, which is then subsequently recorded in marine (e.g. deep-sea fan) sediments from the Republic of Congo
(Spencer-Jones et al., 2015; Talbot et al., 2014; Wagner et al., 2014). More recently, De Jonge et al. (2016) identified aminopen-tol in sediments of the Yenisei River outflow and tentatively proposed that it might indicate decomposition of sub-sea per-mafrost with associated methane release and subsequent mi-crobial oxidation. Here in Buor-Khaya Bay, concentrations of aminopentol (1c) ranged from 0 to 9.5 µg g−1OC, consider-ably lower than values reported in De Jonge et al. (2016, 0–48 µg g−1OC). It is therefore possible that the aminopentol signature is fluvially transported from areas of active aero-bic methane oxidation within the catchment (i.e. polygonal tundra, wetlands, thermokarst lakes) or, alternatively, indi-cates oxidation of methane released from sub-sea permafrost deposits known to be common across the ESAS including Buor-Khaya Bay (Shakhova and Semiletov, 2007; Shakhova et al., 2009, 2010a, b, 2014, 2015).
The ESAS surface sediments generally had only low lev-els of composite BHPs (i.e. BHPs with more complex moi-ety at the C-35 position such as a sugar or amino-sugar; e.g. Rohmer, 1993). The only exceptions were BHT cycli-tol ether (1h), which was found in concentrations ranging from 0 to 49 µg g−1OC, with a mean of 7.7 µg g−1OC(0–10 % of all analysed BHPs), and BHT glucosamine (1i), which was even less common (Table S2). Both of these structures have a wide range of known sources, so they cannot be assigned to any specific group of source organisms (see review in Talbot et al., 2008).
3.2 R0soil, stable carbon isotopes and BIT in ESAS sediments
Bulk carbon isotope values (δ13C) are commonly used as a proxy for marine vs. terrestrial influence on sedimentary OC composition as terrestrial plants using the C3 synthe-sis pathway typically have more depleted values than OC produced via marine primary productivity (Hayes, 1993; Meyers, 1997; van Dongen et al., 2008). Here, we com-pared bulk carbon isotopes to the BHP-based R0soil proxy (Eq. 2). As expected, R0soilvalues were higher closer to the coast and reduced gradually with increasing distance off-shore (Figs. 2c, 3c), including the region off the Indigirka which had somewhat lower absolute concentrations of the individual BHPs compared to the more eastern and western extents of the ESAS region (Fig. 2a, b). The highest values occurred in Buor-Khaya Bay (maximum R0soil=0.80; Ta-ble S2); however, the mean values for Buor-Khaya Bay and Dmitry Laptev Strait sediments were very similar at 0.49 and 0.52, respectively (Fig. 4). The ESAS nearshore sediments had an average only slightly lower at 0.41, whilst the aver-age for the offshore sediments was 0.14 (range 0.42 to 0.00; Fig. 4, Table S2).
A clear negative linear relationship was observed between R0soiland bulk δ13C (r2= −0.73, p < 0.001; Fig. 5a) across the ESAS in agreement with a pilot study of soil micro-bial biomarkers in ESAS surface sediments from the Kolyma
0 40 80 120 160 200 240 280 320 0 40 80 120 160 200 240 BKB DLS Near Off R’soil= 0.8 R’soil= 0.6 R’soil= 0.4 R’soil= 0.2 Bour-Khaya Bay Dimitry Laptev Strait ESAS nearshore ESAS offshore
Non-methylated soil markers (µg gOC-1)
BH T ( µg gOC -1)
Figure 4. Plot of the concentrations (µg g−1OC) of BHT vs.
non-methylated soil markers grouped according to sampling location in Buor-Khaya Bay, Dmitry Laptev Strait, ESAS nearshore and
off-shore. Labelled contours show the R0soilindex values.
River mouth offshore transect where strong linear correla-tions were observed between the R0soil proxy and distance from river mouth (r2=0.97) and bulk δ13C (r2=0.96; Do˘grul Selver et al., 2015). Although bulk δ13C in ESAS surface sediments did display a linear relationship with dis-tance offshore (Vonk et al., 2012), previous comparison of bulk δ13C and BIT values from the ESAS surface sediments displayed a strongly non-linear correlation (Fig. 5c; Sparkes et al., 2015). This was shown to result from a rapid reduc-tion in concentrareduc-tion of brGDGTs in near-coastal sediments (< 150 km from shoreline), causing a drop in BIT values from ∼ 1 to ∼ 0.25 with little variation in bulk δ13C followed by an enrichment in bulk carbon isotopes towards more ma-rine values concomitant with a slower decline in BIT values towards 0 further than 150 km offshore (Sparkes et al., 2015). A similar non-linear relationship is observed here between R0soiland BIT (Fig. 5b), as also previously demonstrated for the surface sediment offshore transect off the Kolyma River mouth (Do˘grul Selver et al., 2015), indicating that similar processes are operating across the entire ESAS.
The simple linear correlation between R0soiland bulk car-bon isotope values is intriguing as it suggests that, unlike the brGDGTs, which in this region are proposed to be pri-marily derived from fluvial transport (De Jonge et al., 2014; Do˘grul Selver et al., 2015; Peterse et al., 2014; Sparkes et al., 2015), the R0soilproxy provides a more integrated signature of different terrestrial sources including ICD and fluvially transported topsoil-permafrost or riverine-produced material. Therefore, soil marker BHPs and brGDGTs, despite being nominally derived from similar sources, i.e. terrestrial mi-crobial membrane lipids, appear in fact to be representing different aspects of terrestrial OC export.
-30 -28 -26 -24 -22 -20 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Buor-Khaya Bay Dmitry Laptev Strait ESAS nearshore ESAS offshore 0 0.2 0.4 0.6 0.8 1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0 0.2 0.4 0.6 0.8 1 -30.0 -28.0 -26.0 -24.0 -22.0 -20.0 R’soil R’soil δ13C ‰ δ 1 3C ‰ B IT in d ex B IT in d ex Lena Delta ICD Kolyma-Indigirka ICD (a) (b) (c) Topsoil-PF ICD-PF ICD-PFa Topsoil-PFa
Kolyma freshet & Yedoma stream Kolyma ICD Kolyma freshet, Yedoma stream Kolyma ICD Branched GDGT reduction Branched GDGT reduction Lena Delta ICD Kolyma-Indigirka ICD
Figure 5. Cross plots of R0soil vs. (a) bulk δ13C (Karlsson et al.,
2011) and (b) BIT index (Sparkes et al., 2015) and (c) BIT index
vs. δ13C in ESAS sediments. Typical values for terrestrial BIT
in-dex vs. δ13C endmembers are indicated (R0soil – this study; δ13C
– Vonk et al., 2012; BIT index – Bischoff, 2013 and Peterse et al., 2014). Note BHP and bulk carbon isotope plot shows linear mix-ing trend, whilst BIT index shows non-linear relationship to both other parameters; the BIT index drops significantly before a shift in
isotope ratio to more marine values or shift to lower R0soilvalues.
R0soilendmember values are lower than 1 due to presence of BHT
in terrestrial materials (Table S3) and typically lower in the Lena River region (mean = 0.50) than in the eastern ESAS region (mean
=0.76).aICD, ice complex deposit; PF, permafrost.
De Jonge et al. (2016) recently demonstrated that soil marker BHPs can indeed be transported in suspended par-ticulate matter (SPM) from the Yenisei River, a large river located west of the ESAS. In the Yenisei study, only a mod-erate correlation was observed between the δ13C values and R0soil (r2=0.44, p < 0.01), suggesting they trace different pools of OM (bulk terrigenous OM versus bacterial OM).
However, unlike in the current study, De Jonge et al. (2016) also found a strong linear correlation between R0
soiland BIT (r2=0.82, p < 0.05). The Yenisei River catchment and out-flow have significantly different characteristics to those of the rivers entering the ESAS, specifically in terms of the extent of different permafrost regimes. Permafrost is classi-fied as isolated, sporadic, discontinuous and continuous ac-cording to its spatial distribution (e.g. Zhang et al., 2008). Continuous permafrost means that over 90 % of the area is frozen in contrast to discontinuous permafrost, where only 30–80 % of the area is underlain by permafrost. The Yeni-sei drains an area with a high proportion of discontinuous permafrost (55 %), whilst the proportion of continuous per-mafrost is lower (33 %; Feng et al., 2013, and references therein). The proportion of continuous permafrost in the east-ern river catchments is higher, ranging from 79 % (Lena) to 100 % (Kolyma and Indigirka; Feng et al., 2015; and refer-ences therein). This may in turn affect the composition and preservation of terrestrial microbial markers. Indeed, previ-ous work has indicated that OC from continuprevi-ous permafrost areas is older but also less degraded and more biolabile than that from areas of discontinuous permafrost (e.g. Cooke et al., 2009; Feng et al., 2015; Mann et al., 2015; Spencer et al., 2015; van Dongen et al., 2008). Additionally, although there is some evidence of coastal erosion of ICD in this re-gion, such as in the Khalmyer Bay area, this is not directly drained by the Yenisei River. Therefore, in this more westerly region it is likely that the primary source for both soil marker BHPs and brGDGTs is fluvial transport, hence the linear re-lationship between soil microbial biomarkers and bulk δ13C values (De Jonge et al., 2015, 2016). Whereas in the more easterly region we assume brGDGTs to be of fluvial origin (Sparkes et al., 2015) and the soil marker BHPs to have an integrated signature of terrestrial sources including ICD and fluvially transported topsoil-permafrost or riverine-produced material.
3.3 Terrestrial endmembers and implications for ESAS sedimentary carbon budgets
Given the apparent discrepancy between the BHP and GDGT-derived signals, further consideration of the terres-trial endmembers is clearly required. Although extensive databases exist for different bulk isotopic endmembers for the Arctic region (e.g. Tesi et al., 2014; Vonk et al., 2012; and references therein), data on the soil microbial lipid de-rived proxy values are limited for the Siberian region (see Table 1 for summary). Recently, Peterse et al. (2014) re-ported high BIT values for a range of materials from the Kolyma region (eastern ESAS), including thermokarst and floodplain lake sediments (BIT = 1), yedoma (and associated streams; BIT = 0.81–0.89) and SPM from the Kolyma River including samples collected during the spring freshet (BIT = 0.99–1). However, Sparkes et al. (2015) reported values for three ICD (Yedoma) samples from the same area
rang-ing from 0.44 to 0.7. The lower values resulted from rela-tively high levels of crenarchaeol, which is unusual for ter-restrial materials (typical BIT values > 0.8; Schouten et al., 2013), although this compound has been reported from sev-eral Thaumarchaeota isolated from soil (Sinninghe Damsté et al., 2012). Data on BHPs from the (East Siberian) Arc-tic region are scarce as previous studies of terrestrial BHPs have primarily focused on temperate and more recently on tropical regions (see review in Spencer-Jones et al., 2015). Do˘grul Selver et al. (2015) reported average R0soil values of 0.76 (range 0.70–0.84) for the same three ICD yedoma samples from the Kolyma region (CHYED-2; Table 1, Ta-ble S3). Höfle et al. (2015) also reported the BHP composi-tion in polygonal active layer deposits (to a maximum depth of 48 cm) from two locations in the Lena Delta, Samoylov Is-land and Kurungnakh IsIs-land. Calculating R0soil values from these data revealed a wide range of values from 0.18 to 0.79 and a mean average of 0.41 (Table 1). Although this sim-ple average will likely not represent a spatially and depth resolved average for the region, it is still close to the mean values found in Buor-Khaya Bay and Dmitry Laptev Strait sediments (Table S2).
To further evaluate potential endmember ranges, addi-tional ICD samples from the Lena, Indigirka and Kolyma re-gions were investigated for BHPs (Table S3). We analysed 23 samples from a 25 m permafrost core from Kurungnakh Island (see details in Bischoff et al., 2013) and an additional sample from Cape Bykovsky at 1.9 m depth (Table S3). BHT and a range of soil marker BHPs were present in all samples. BHT concentration ranged from 7 to 643 µg g−1OC(mean av-erage 248 µg g−1OC) in the Kurungnakh Island permafrost de-posits. As in the ESAS sediments (Table S2), the soil marker BHPs included high proportions of adenosylhopane (1a), fol-lowed by adenosylhopane type 2 (1b) and adenosylhopane type 3 (1b’) (mean average 250 µg g−1OC; Table S3). As ex-pected, observation of the methylated compounds was inter-mittent and then only at very low levels (Table S3), justify-ing their exclusion from R0soil (Do˘grul Selver et al., 2012, 2015). R0soil values ranged from 0.37 to 0.64 with a mean value of 0.50 (Table 1), whilst the Cape Bykovsky (CB) sam-ple had an R0soilof 0.68 (Table 1). The low R0soil values in ICD from this region (0.34 to 0.80, mean 0.49; Figs. 3c, 4) are in excellent agreement with the mean and range of val-ues found within the ISSS-08 Buor-Khaya Bay and Dmitry Laptev Strait sediments (0.49 and 0.52 respectively; Ta-ble S2; Fig. 5a). Although additional sources of BHPs from fluvial transport and from material transported via changes in hydrological conduits resulting from thermokarst erosion are also possible (POC; e.g. Vonk et al., 2015), there are cur-rently no data on the BHP composition in this fraction from this region for comparison.
Bulk δ13C was not measured for the KUR core used in this study, but Wagner et al. (2007) reported values between −23.1 and −24.6 ‰ for the OC fraction of selected
per-Table 1. Mean, maximum and minimum values for OM proxy values (R0soil, BIT and δ13C) by sample group location and type of material.
Location na R0soil R0soil R0soil na BIT BIT BIT na δ13C δ13C δ13C
(mean) (max) (min) (mean) (max) (min) (mean) (max) (min)
‰ ‰ ‰
ISSS-08 sediments
Buor-Khaya Bay 47 0.49 0.80 0.34 47b 0.58 0.95 0.26 37c −25.9 −25.3 −26.6
Dmitry Laptev Strait 7 0.52 0.57 0.48 7b 0.55 0.66 0.46 7c −27.2 −26.9 −27.4
ESAS nearshore 9 0.41 0.57 0.27 9b 0.35 0.58 0.10 8c −26.8 −26.2 −27.4
ESAS offshore 29 0.14 0.42 0.00 29b 0.06 0.28 0.00 28c −24.2 −21.2 −26.5
Ice complex
Lena Delta (KUR) 23 0.50 0.64 0.37 23d 0.97 1.0 0.87 23 n.d.e n.d. n.d
Cape Bykovsky 1 0.68 1 n.d. 1 −26.0
Kolyma+Indigirka 11 0.76 0.84 0.62 3b 0.53 0.7 0.44 5 −24.3 −23.02 −25.8
Literature data
ICD permafrost 374c −26.3 ± 0.7f
Topsoil permafrost 20c −28.2 ± 2.0f
Yedoma (Duvanny Yar) 1g 0.82
Yedoma stream (Duvanny Yar) 8g 0.83 0.89 0.81
SPM (Kolyma River) 6g 1.0 0.99
Lena Delta permafrost soils 24h 0.41 0.79 0.18 an, number of samples used for calculation of mean for individual parameters; bdata from Sparkes et al. (2015);
cdata from Vonk et al. (2012); ddata from Bischoff (2013) en.d., not determined; fmean ± standard deviation; gdata from Peterse et al. (2014); hdata from Höfle et al. (2015).
mafrost sediments from Samoylov Island, which lies close to Kurungnakh Island in the Lena Delta (see map in Höfle et al., 2015). These values from Samoylov Island are signif-icantly enriched relative to the value for the Cape Bykovsky sample (−26.0 ‰; Table S3) and may be because these de-posits are genetically different (Holocene fluvial sediments vs. Pleistocene ICD) or reflect input from aquatic plants in low-centre polygon ponds (Schirrmeister et al., 2011). How-ever, input from peat, grasses, herbs and shrubs results in more negative values such as those reported by Schirrmeis-ter et al. (2011) for a range of sites in the region includ-ing Kurungnakh Island and the Bykovsky Peninsula (range −30 to −25 ‰). The δ13C values for the ISSS-08 sediments from Buor-Khaya Bay (−25.3 to −26.6 ‰) and the Dmitry Laptev Strait (−26.9 to −27.4 ‰) therefore suggest a signif-icant contribution of terrestrial material. Vonk et al. (2012) reported an extensive compilation of circum-Arctic literature data with an average δ13C value of −26.3 ± 0.7 ‰ for ICD OC (coastal, inland and sub-sea; formed before inundation) and even more depleted values for topsoil permafrost with δ13C of −28.2 ± 2.0 ‰ (Table 1). By combining bulk13C and14C data, these authors estimated the proportion of sedi-mentary OM derived from ICD, topsoil and marine sources, with over two-thirds of the OM in Buor-Khaya Bay derived from terrestrial sources and even higher values in Dmitry
Laptev Strait sediments. These terrestrial OM estimates can now be compared to estimates based on the BHP concentra-tions/compositions obtained using a similar approach with the results from the ICD samples as the terrestrial endmem-bers.
ICD samples from the Indigirka (n = 3) and Kolyma River (n = 8) regions had lower absolute concentrations of BHPs (BHT range 8.5 to 62 µg g−1OC; non-methylated soil markers range from 43 to 123 µg g−1OC) and generally higher R0soil val-ues than those from the Lena region (mean 0.76, range 0.62– 0.84; Tables 1, S3; Fig. 5a). Although there are some differ-ences in the relative abundances of the non-methylated soil markers BHPs between the different regions (Table S4), in all cases BHP 1a was the most abundant, followed by BHP 1b and minor relative amounts of BHP 1b’ (see Table S4). Correlation of concentrations of all three pairs of these non-methylated soil markers (BHP 1a vs. BHP 1b, BHP 1b vs. BHP 1b’ and BHP 1a vs. BHP 1b’) in all ICD samples gives an r2>0.6 and p value < 0.001 for each pair of compounds. This shows that the distribution of the non-methylated soil markers in all ICD samples is comparable across the East Siberian Arctic region. Given the higher R0soilvalues for ICD in the eastern region, this suggests that although some re-moval of soil marker BHPs may have already occurred in the river estuaries before reaching the near-coastal shelf
sed-iments, a significant proportion remains intact. Based on the range of ICD permafrost endmember data for the Kolyma and Indigirka samples, this would suggest that 58 to 92 % of the OC derived from terrestrial OM (e.g. % ICD and top-soil) at the start of the offshore Kolyma River mouth surface sediment transect (sample YS-34B, R0soil=0.57) and 49 to 77 % of the OC derived from terrestrial OM at the start of the offshore transect off the Indigirka River mouth (sample YS-30, R0soil=0.48) is of terrestrial origin. These values should be treated with caution given the limited number of samples involved, but are close to the average value reported for the ESAS surface sediments based on dual-carbon-isotope (δ13C and 114C) mixing models (75 ± 14 % from terrestrial origin; Vonk et al., 2012).
This difference in R0soil values in permafrost de-posits/ICDs across the East Siberian Arctic region is likely due to two separate regional provinces, both underlain by mostly continuous permafrost, but with permafrost deposits of different ages and formation mechanisms (Schirrmeister et al., 2011). Further, samples in the western and far eastern region are subject to different environmental conditions (with colder and drier conditions in the far eastern region (Gordeev, 1996), potentially indicating better preservation of the struc-turally more complex soil marker BHPs relative to BHT (see Sect. 3.2). This agrees with previous studies which have also found better preservation of more highly functionalised and biolabile molecules in materials from the eastern region (e.g. Guo et al., 2004; van Dongen et al., 2008). Tesi et al. (2016) reported that fine and ultrafine grain size fractions contain a high proportion of high-molecular-weight lipid markers which are preferably bound to the mineral matrix and that the reactivity of lipid biomarkers on the ESAS seems to be lower and inversely proportional to the number of functional groups (cutin acids > alkanolic acid > alkanols > n-alkanes). Even though the reactivity for different BHPs is currently unknown, this points towards a potential recalci-trance of highly functionalised BHP molecules on the ESAS. Furthermore, studies from soils have shown the potential for mineral–organic interactions, leading to increased resis-tance to degradation for aromatic compounds (Mikutta et al., 2007, 2009). Adenosylhopane, the most abundant single soil marker BHP on the ESAS, is the only BHP containing an aro-matic moiety (adenine). Although Höfle et al. (2013) found organic–mineral associations to be of minor importance in the polygonal tundra of the Lena Delta, organic–mineral in-teractions may still be among several factors explaining the high relative abundance of these compounds under certain conditions.
However, the current sample set discussed is limited given the enormous spatial scale and extremely heterogeneous na-ture of these environments and does not include, for exam-ple, material released from thermokarst environments, in-cluding thermokarst lake sediments, which can be an impor-tant source of OC and inorganic material (Vonk et al., 2015). Furthermore, environmental parameters other than location
(and inferred temperature) must also be considered as po-tentially affecting the overall BHP assemblage. For exam-ple, Höfle et al. (2015) recently demonstrated using principal component analysis that increasing pH, over a range of 4.5 to 6.7, was positively correlated with soil marker BHP concen-tration in Lena Delta permafrost, which is in close proximity (< 10 km) to the site of the KUR core studied here. These authors proposed that this might indicate source organisms do not need to further extend their BHP side chains to alter membrane architecture at near-neutral conditions. Further-more, studies of temperate Sphagnum peat deposits, which typically have low pH values, show low abundance of soil marker compounds relative to total BHPs when compared to mineral soils, but have higher abundance of BHT result-ing in very low R0soilvalues (mean 0.4; calculated from data in van Winden et al., 2012a, b). This could suggest that, in areas with a significant input from peat-derived material in some areas/layers of the Lena Delta ICD (Bischoff et al., 2013; Wagner et al., 2007), lower R0soil values should be expected in agreement with observations in this study (Ta-ble S3). Indeed, pH has also been shown to play a domi-nant role in shaping bacterial communities with the capac-ity to produce hopanoids in an acidic peatland (Gong et al., 2015). Clearly a more comprehensive assessment of differ-ent terrestrial endmembers across the region is required, as are additional studies on primary environmental factors af-fecting BHP biosynthesis in culture. For example, to date no studies have investigated biosynthesis of adenosylhopane in psychrophilic/psychrotolerant organisms at different temper-atures or at different growth stages, and studies of BHPs in association with pH adaptation have yet to measure adeno-sylhopane abundances (e.g. Welander et al., 2009). Given that adenosylhopane is the precursor for biosynthesis of all other side-chain extended BHPs (Bradley et al., 2010), it is possible that, under conditions of stress (such as extreme temperature and nutrient limitation), production of adenosyl-hopane without further modification is sufficient and/or all that some organisms are capable of and further modification is metabolically unfavourable/unnecessary.
4 Conclusions
Different suites of terrigenous microbial membrane lipids (biohopanoids and brGDGTs) and bulk carbon isotopes were used to trace the source and transport of terrestrial OC on the ESAS. As expected, ESAS sediments are terrestrially dom-inated; however, BHP- and GDGT-based SOM proxies are decoupled in Buor-Khaya Bay, southeastern Laptev Sea and across the ESAS, in agreement with an earlier pilot study of the surface sediment offshore transect off the Kolyma River mouth (Do˘grul Selver et al., 2015). This is likely due to dif-ferent sources, transport and/or degradation pathways for the various lipids. In particular, whilst brGDGTs have previously been shown to be primarily delivered to the ESAS via fluvial
transport (Sparkes et al., 2015), BHPs appear to provide a more integrated signature correlating linearly with bulk car-bon isotope ratios as well as distance from river mouths. The BHP terrestrial endmembers, i.e. adenosylhopane and other soil marker BHPs, are significant components of coastal ICD which are transferred to ESAS sediments during coastal ero-sion. R0soilproxy values, although still limited for this region, vary widely with on average significantly lower values oc-curring to the western range of East Siberia (average 0.5 for the Lena Delta ICD) and higher values further east (average 0.76 for Indigirka and Kolyma ICD). The controlling fac-tors responsible for this difference may include facfac-tors such as depositional history and age of permafrost deposits, min-eral grain size, environmental/abiotic factors (e.g. tempera-ture, precipitation and pH), and microbial/metabolic factors under psychrophilic conditions and require further investiga-tion.
5 Data availability
Data used in this article can be found in the Supplement.
The Supplement related to this article is available online at doi:10.5194/bg-13-4899-2016-supplement.
Author contributions. Ö. Gustafsson, B. E. van Dongen, O. V.
Du-darev, and I. P. Semiletov collected samples along with the crew of ISSS-08. Ice complex samples were collected and provided by R. G. M. Spencer, E. Rivkina, D. Wagner and A. N. Kurchatova. H. M. Talbot and B. E. van Dongen designed the study, which was carried out by J. Bischoff, with assistance from R. B. Sparkes and A. Do˘grul Selver. H. M. Talbot, J. Bischoff, R. B. Sparkes and B. E. van Dongen prepared the manuscript with contributions from all co-authors.
Acknowledgements. We gratefully acknowledge receipt of a
NERC research grant (NE/I024798/1 and NE/I027967/1) to B. E. van Dongen and H. M. Talbot, a Ph.D. studentship to A. Do˘grul Selver funded by the Ministry of National Education of Turkey, financial support as an Academy Research Fellow to Ö. Gustafsson from the Swedish Royal Academy of Sciences through a grant from the Knut and Alice Wallenberg Foundation and support from the Government of the Russian Federation (grant #14, Z50.31.0012/03.19.2014) to I. Semiletov and from the Russian Scientific Foundation to O. Dudarev (grant # 15-17-20032). We thank the crew and personnel of the R/V Yakob Smirnitsky and all colleagues in the International Siberian Shelf Study (ISSS) programme for support, including sampling. We thank A. N. Kurchatova for assistance with fieldwork on Kurungnakh Island and T. Tesi for providing the Yedoma samples for the Kolyma and Indigirka catchment areas. We thank P. Lythgoe (University of Manchester) and F. Sidgwick (Newcastle University) for invaluable
assistance with LCMS and the Science Research Investment Fund (SRIF) from HEFCE for the Thermo Finnigan LCQ ion trap mass spectrometer (Newcastle University). R. G. M. Spencer was partially supported by the U.S. National Science Foundation (ANT-1203885/PLR-1500169). The ISSS programme is supported by the Knut and Alice Wallenberg Foundation, the Far Eastern Branch of the Russian Academy of Sciences, the Swedish Research Council, the US National Oceanic and Atmospheric Administration, the Russian Foundation of Basic Research, the Swedish Polar Research Secretariat, the Nordic Council of Ministers and the US National Science Foundation. Finally, we thank the associate editor and the two anonymous reviewers for constructive suggestions.
Edited by: J. Middelburg
Reviewed by: two anonymous referees
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